Current classical theory of radiation reaction has several deficiencies such as "runaway solutions" and violation of causality. The Landau-Lifshitz approximation to the exact equation introduced by Lorentz, Abraham and Dirac is widely used, though questions remain regarding its domain of validity. This thesis explores an alternative treatment of the motion of a radiating electron, based on an equation first proposed by Ford and O'Connell. A general condition is found for solutions of this equation to deviate from those of Landau-Lifshitz. By exploring radiation reaction effects on a particle colliding with an ultra-intense laser pulse we show that the regime where there is a significant deviation of these two approaches can never be reached with existing or proposed laser facilities. The methods used to explore single particle interaction with an intense laser pulse are extended to describe the interaction of a particle bunch with various realistic laser pulses. We find that the interaction leads to a decrease in average momentum and relative momentum spread. However, the decrease appears to be independent of the length of the pulse and depends only on the energy in the pulse regardless of how it is distributed. Radiation reaction effects occuring during the scattering of an electron by a heavy, highly-charged nucleus are studied. Radiation reaction is seen to affect the particle's motion. We find noticeable differences between the predictions of the Ford-O'Connell and Landau-Lifshitz equations, albeit in regimes where quantum effects would be important.

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Ion and antiproton beams at keV energies are very attractive for a number of fundamental studies. The diagnostic of these beams, however, is a challenge because most detectors are either not sensitive enough or too disturbing. In this work, a set of optimised detectors has been developed to suit the particular beam diagnostic needs of the novel Ultra-low-energy Storage Ring (USR) at the future Facility for Low-energy Antiproton and Ion Research (FLAIR), accommodating as few as 10^7 particles at energies down to 20 keV. The instrumentation includes beam profile monitors based on scintillating screens and secondary electron emission, ultra-sensitive Faraday cups for absolute intensity measurements, and capacitive pick-ups for on-line beam position monitoring. This thesis presents results from theoretical studies and modelling, the design of prototypes of all monitors, and results from measurements with beam. All detectors are characterised in detail, their individual limitations are described and options for further improvements are indicated. Whilst initially developed for the USR, the instrumentation described in this thesis is also well suited for use in other low-intensity, low-energy accelerators, storage rings, and beam lines.

Excited states in 155Ho and 156Er extending to high spin have been populated as part of two experiments performed at the atlas facility of Argonne National Laboratory. In both experiments high-fold gamma-ray coincidence data were recorded in a high-statistics experiment with the Gammasphere spectrometer. In 156Er, weakly populated band structures have been established at low to medium spins, following the 114Cd(48Ca,6n) reaction at 215 MeV. Bands built on the second 0+ state and 2+ (gamma-vibrational) states have been established. A large staggering between the even- and odd-spin members of the gamma-vibrational band suggests a gamma-soft nature of this nucleus. An additional band is discussed as being based on a rotationally aligned �h9/2/f7/2 structure, coexisting with the systematically observed, more favourable (�i13/2)2 alignment seen in this mass region. At high spin, new transitions above the 42+ terminating state have been observed, establishing the presence of a further excited state, which is interpreted as the predicted terminating 46+ state built on a �particle-hole excitation (d5/2/g7/2 to d3/2) across the Z = 64 shell gap. New states in 155Ho have been observed following the 124Sn(37Cl,6n) reaction at 180 MeV. At low spin, a positive-parity �g7/2 rotational band, observed for the first time in this nucleus, has been established up to (33/2+). The �h11/2 band has been extended to higher spins, and the large signature splitting it exhibits interpreted as evidence of triaxiality at low spin. In addition, a positive-parity rotational structure has been extended to I� = 57/2− and discussed as being based on a �(i13/2h9/2) structure coupled to the h11/2 proton. B(M1)/B(E2) ratios of reduced transition probabilities have been calculated from observed branching ratios for all three bands and compared with theoretical calculations for specific particle configurations. Above spin � 59/2 the behaviour of the nucleus is discussed in the context of band termination. An energetically favoured state at I� = 79/2− has been established, which corresponds to valence-space band termination in 155Ho. Weak, high-energy core-breaking excitations feeding this state have been observed. New states with I� = (87/2+), (89/2+) are thought to be further predicted favoured terminating states at oblate shape, built on �(d5/2/g7/2 ! h11/2) particle-hole excitations across the Z = 64 shell gap. The newly observed high-spin states are compared to Cranked Nilsson Strutinsky calculations, and presented in the context of known band terminating states in the region.

The Tokai-to-Kamioka (T2K) experiment is a second generation long-baseline neutrino experiment and the first to use an off-axis neutrino beam to produce narrow neutrino energy spectrum. T2K was designed to measure with precision the atmospheric mixing parameters, and also look for evidence of non-zero θ13. T2K's near detector (ND280) provides constraints on the beam flux and neutrino cross-section uncertainties, as well as making valuable cross-section measurements. This thesis describes an oscillation analysis that uses samples from both near and far detectors. Importantly, vμ and Ve samples at the far detector are combined to produce a joint oscillation analysis. A Markov chain Monte Carlo is used to construct the Bayesian posterior distribution by sampling a likelihood function. From the Bayesian posterior distribution, the oscillation parameters of interest and their errors are estimated.

Gamma-ray spectroscopy is an essential tool in the study of nuclear phenomenon. The study of exotic nuclei and nuclear states have been used to expand the nuclear chart as well as understand the origin of the universe. Large volume high purity germanium arrays, very high beam intensities and more recently exotic beams have lead to new understanding of nuclear physics. The Advanced Gamma Tracking Array (AGATA) aims to utilise high purity germanium (HPGe) detectors in order to achieve a dramatic increase in efficiency over current spectrometers. The work detailed in this thesis shows a highly detailed characterisation of two AGATA asymmetric capsules of the same shape in order to test and compare performances. Detector A004 was acceptance tested and scanned at the University of Liverpool in February 2010. Detector A006 was scanned between April and September 2010. Resolution, efficiency and charge collection parameters have been studied, comparing these two detector. The results of the comparison show an excellent agreement between the performance of the two detectors. The depletion rate of the detectors has also been compared quantitatively indicating the dependence of depletion on impurity concentration of the capsules. An experimental pulse shape database was generated for detector A006. The sensitivity of the detector response was quantified indicating the regions of the detector of high variation in pulse shape response. The AGATA detector library, which used to simulate the detector response for pulse shape analysis, is compared with this database. The position resolution achieved between the two data sets has been calculated. The combination of the detailed comparison and characterisation of the detector with the study of the simulate database will provide the AGATA collaboration with useful information to improve detector simulations in the further for pulse shape analysis.

A pre-data sensitivity study to search for a high mass Standard Model (SM) Higgs (m_H = 200 GeV) in ATLAS using the H -> ZZ -> llll channel is presented. It is found that it would be possible to exclude a SM Higgs in part of this high mass region with limited luminosity. Using this channel a search at the LHC for the SM Higgs boson in the first ~40 inverse picobarns of data was conducted and is presented in this thesis, along with the results from the H->ZZ->llvv and H->ZZ->llqq channels using a similar dataset. It is found that the channel with the best sensitivity to a SM Higgs with mass greater than 200 GeV is the H->ZZ->llll channel. A search for the SM Higgs boson using the H->ZZ->llvv channel is presented, using 4.7 inverse femtobarns of data at a centre-of-mass energy of s = 7 TeV. A Higgs boson with a mass between 320 and 560 GeV is excluded at a 95\% confidence level using this channel alone. This analysis was published in 2011 and updated in 2012, and results from this search are used in the ATLAS paper, describing the discovery of a new Higgs-like boson with ~m_H = 125 GeV. Finally a direct search is performed for anomalous invisible decays of the Higgs boson candidate at ~125 GeV using both the 2011 dataset and the 2012 dataset at centre-of-mass energy s = 7 and 8 TeV respectively. An upper limit of 65% is set on the allowed H->inv branching fraction at 95% confidence level. Additional searches are performed using the same dataset on further invisibly decaying Higgs-like bosons at masses between 115 and 300 GeV. No excess is observed. This analysis was published as a preliminary result in March 2013, and a paper using the full 2011 and 2012 datasets is scheduled to be published in the summer of 2013.

In an e�ort to gain a fundamental understanding of the link between catalyst surface structure, reactivity and stability, experiments have been performed on Pt(111), Pt3Ni(111) and Pt(100) single crystal electrode surfaces via a combina- tion of in-situ structural probes and electrochemical (cyclic voltammetry) mea- surements. Pt(111), which is the least active of the low index surface for the Oxygen Reduction Reaction (ORR), showed morphological changes after oxida- tion reduction cycles, with the formation of stable platinum adislands. The more active Pt(100) surface undergoes a drastic roughening of the surface during ox- idation/reduction cycles, which is due to the fact that the surface is unlikely to recrystallise into the more open (100) surface structure. In essence, although the Pt surfaces with a more open atomic geometry, e.g. (100) and (110), are more reactive than the close-packed (111) surface, they are intrinsically less stable. The bimetallic alloy Pt3Ni(111) surface has been shown to be over 10 times more ac- tive for the ORR than pure Pt(111) and exhibits a compositional oscillation at the surface which is formed of Pure Pt. Upon repetitive cycling the Pt3Ni(111) surface undergoes a similar morphological change to that of Pt(111), where the formation of adislands was seen in STM measurements. These adislands retain some of the active segregated surface structure, preserving the electronic modi�cation due to the Ni-rich subsurface. This leads to a stable surface that remains more active than the pure Pt(111) electrode after prolonged cycling of the potential into the region of oxidation/reduction. Perhaps the key insight that can be obtained from these experiments is that electrode activity towards the ORR is intrinsically linked to surface stability, i.e. the more active the surface, the less stable it is. This, in fact, may be a general e�ect in electrocatalysis.

The Advanced GAmma Tracking Array (AGATA) project is an European collabora- tion project that is aiming to construct a next generation gamma-ray spectrometer for nuclear-structure applications. The final array will consist of 180 highly-segmented HPGe detectors mounted into 60 triple clusters forming a full 4π shell. AGATA will utilise pulse-shape analysis and gamma-ray tracking to achieve high photopeak efficiency and peak-to-total. Central to the AGATA spectrometer is the Nouvelle Ac- quisition temps-Réel Version 1.2 Avec Linux (NARVAL) data-acquisition system that will perform pulse-shape analysis and gamma-ray tracking calculations in real time (online). The system is comprised of separate blocks of actions, called actors. Each one containing a self-sufficient piece of code designed for a specific action. AGATA data can be downloaded and replayed through an offline version of NARVAL, known as the NARVAL-emulator. With a new format and techniques there is need to define new methodology to analyse the data. Discussed in the first part of this work will be processing of AGATA data and how by utilising the actors of the NARVAL-emulator different output formats can be chosen. In the next part of this work, the Compton polarimetry capability of an AGATA triple cluster will be investigated by utilising pulse-shape analysis and the detector’s granularity. The basic design of a Compton polarimeter consists of two detectors; one acting as a scatterer and the second as a absorber. The same principles apply to an AGATA triple cluster with each sector act- ing as the scatterer and absorber. The pulse-shape analysis algorithm identifies the positions and energies of the interaction points. Data was obtained from the heavy- ion fusion-evaporation of 110Pd (32S,4n) 138Sm at 135 MeV was used to investigate the Compton polarimetry capability of an AGATA triple cluster. This work utilised the phenomenon that the scattering cross-section for gamma rays is larger in the direction perpendicular to its electric field vector than parallel. This is described by the differ- ential Compton scattering cross-section for polarised gamma rays. A useful event to determine linear polarisation occurs if the incident gamma ray is Compton scattered from the first interaction and then fully absorbed at the second interaction. The sum of the energies deposited in the two interactions gives the energy while the azimuthal scattering angle contains the linear polarisation information. To determine the linear polarisation of a gamma ray, a polarisation sensitivity and asymmetry measurement can be made. Such measurements were made in this work using gamma rays from 138Sm of known linear polarisations. The polarisation sensitivity method utilised the incident and scattered gamma-ray energy, in addition to the azimuthal scattering an- gle information produced from the pulse-shape analysis algorithm. The experimental polarisation sensitivities ranged from 5.0±0.3% at 139.7 keV to 16.4±0.1% at 1287 keV. A asymmetry measurement was carried by determining the number of gamma rays that scattered perpendicular and parallel to the reaction plane. A positive asym- metry would indicate an electric transition and a negative a magnetic one. It was found that there was partial consistency with expectation. From the asymmetry and polarisation sensitivity measurement an experimental linear polarisation for each gamma ray was determined. The result should be in the range −1≤P(θγ)≤+1 with the sign propagating through from the asymmetry measurement. Finally a coinci- dence efficiency and figure of merit was measured and are given as 32.6±1.0% and 3.9±0.2%, respectively at 346.9 keV. Overall it was concluded that there was limited Compton polarimetry capability and proposed reasons for this were discussed.

Excited states of the proton-unbound nucleus ¹⁷⁷Au have been populated following the ⁹²Mo(⁸⁸Sr,2pn) reaction in an experiment performed at the University of Jyväskylä Accelerator Laboratory. Gamma rays detected in the JUROGAM spectrometer were correlated with the characteristic α decays of ¹⁷⁷Au detected following a recoil implantation in the GREAT spectrometer. A large number of transitions (~ 60 γ rays) have been assigned unambiguously to ¹⁷⁷Au and ordered into four collective band structures and other single-particle excitations. Several intruder configurations formed by single-proton excitations across the Z = 82 shell gap have been established. These have been interpreted as πi13/2, πf7/2 ⊕ h9/2 and πh9=2 prolate configurations. It has been established that the πi13/2 and πf7/2 ⊕ h9/2 bands have decay paths to both the positive-parity mixed (1/2⁺ ⊕ 3/2⁺) ground state and the negative-parity 11/2⁻ isomer, which is unusual in this mass region. Structures based on the coupling of the odd π h⁻¹ 11/2 proton hole to excitations of the ¹⁷⁸Hg core have been established in the excitation level scheme. An oblate πh⁻¹ 11/2 ⊗ ¹⁷⁸Hg(2⁺₁) state and a prolate strongly coupled band based on the πh⁻¹ 11/2 ⊗ ¹⁷⁸Hg(0⁺₂) configuration have been identified. The decay paths from the strongly coupled band and the search for evidence of electric monopole decays are discussed.